One of the primary components in paint is the "film former" that provides a film for the protective function for a substrate coated with the paint. Film forming components of liquid paints include resins which have required organic solvents to provide the resins with suitable viscosities such that the paint can be applied by existing commercial application equipment. Use of organic solvents, however, raises at least two problems. In the past and potentially in the future, petrochemical shortages mitigate against the use of organic solvent in great volumes. Second, environmental concern mitigates against the use of organic solvents and requires such use be minimized.
Environmental concern has become increasingly important. This concern not only extends to preservation of the environment for its own sake, but extends to public safety as to both living and working conditions. Volatile organic emissions resulting from coating compositions which are applied and used by industry and by the consuming public are not only often unpleasant, but also contribute to photochemical smog. Governments have established regulations setting forth guidelines relating to VOCs which may be released to the atmosphere. The U.S. Environmental Protection Agency (EPA) has established guidelines limiting the amount of VOCs released to the atmosphere, such guidelines being scheduled for adoption or having been adopted by various states of the United States. Guidelines relating to VOCs, such as those of the EPA, and environmental concerns are particularly pertinent to the paint and industrial coating industry which uses organic solvents which are emitted into the atmosphere.
To reduce organic solvent content and VOCS, researchers have developed high solids coating compositions, water-borne coating compositions and powdered coating compositions. High solids compositions generally are liquid and are designed to minimize solvents. Powdered coating compositions are solid powders and generally eliminate solvents. Water-borne coating compositions use water to reduce VOCs. While each have advantages, each coating composition has disadvantages.
Coating compositions which include high solids polymeric vehicles based upon polyesters have become popular. Typically these compositions require the use of some organic solvents. In high solid polyesters as opposed to "conventional" compositions which use organic solvents, high molecular weight generally needs to be achieved during crosslinking rather than being attained from the basic polyester polymer. Hence, high solids polyesters normally supply a greater number of reactive sites (predominantly hydroxyl groups) available for crosslinking. The resultant polymers typically exhibit 70-80% solids-weight when reacted stoichiometrically with isocyanate crosslinkers, but frequently yield empirical solids up to 12% lower, when crosslinked with melamine resins. Despite their reduced use of organic solvents, high solids polyester coating compositions can be produced on the same equipment and are employed in many of the same applications as lower solids "conventional" polyester coating compositions. Further, as a result of their many strengths such as ease of manufacturing and use, low volatile emissions, reduced energy requirements, greater application efficiency, lower handling and storage costs, and excellent physical properties, high solids polyester coating compositions have enjoyed spectacular growth in manufacture and use. They still require organic solvents, however, and are a source of VOCs.
Powder coatings and UV-curable coatings are desirable ultrahigh or 100% solids coatings. However, there are limitations as to the technique and the equipment which are used to apply the powdered composition.
Another way to reduce VOCs is to use water as a medium for the film-forming components in the polymeric vehicle. One way is to make a polymer such as a polyester, alkyd, acrylic or epoxy polymer having carboxyl groups and acid number in amounts effective such that when the carboxyl groups are neutralized, they will permit the polymer or oligomer to be dispersed in a mixed solvent system which includes an organic solvent and water. The amine salt of the oligomer or polymer disperses into the mixed water/organic solvent system with the formulation of a dispersion as opposed to a solution. This is a water reducible system.
Another way to reduce VOCs using water is to actually make a solution of polyols which may be oligoesters or acrylic oligomers having hydroxyl groups and a number average molecular weight of less than about 10,000. The hydroxyl number of the oligomer is sufficiently high and effective for permitting the formation of a solution of the oligomer in water or a mixed solvent system of water and organic solvent. This is a water-thinnable polymeric vehicle.
An object of the invention is to provide a polymeric vehicle which will reduce or eliminate VOCs.
Another object of this invention is to provide a hardener for use in a water-reducible or water-thinnable polymeric vehicle or formulated coating composition.
Another object of this invention is to provide polymeric vehicles which are not only low in VOCs, but which provide coating binders with good film properties such as hardness and impact resistance.
Yet another object of this invention is to control the viscosity to low levels at a specific shear rate of a liquid polymeric vehicle or liquid formulated coating composition with the use of water and with the minimization of organic solvents for such control.
Further, objects and advantages of the invention will be found by reference to the following description.